A pellet is generally employed as a material for resin molding or the like. The pellet is prepared by heating/melting a previously blended/adjusted resin raw material for extruding the same into the form of a strand (string) and cutting the obtained resin strand into the form of a small tablet. When molding various resin products or the like, the aforementioned pellet is so prepared that the raw material may not be blended/adjusted every molding of the resin products, a stable operation can be performed, the qualities of the products are stabilized and handling such as an operation of supplying the raw material to molding equipment etc. is simplified.
While the pellet for resin molding is generally entirely molded from the same resin, a multilayer pellet or a composite pellet consisting of a plurality of material portions is also proposed. Patent Document 1 (Japanese Patent Laying-Open No. 07-171828) describes a multilayer pellet of a core-sheath structure having a core of a material rich in viscosity and a sheath of crystalline polyolefin resin having less viscosity for solving such a problem that a pellet prepared from a material rich in viscosity easily causes blocking.
Patent Document 2 (Japanese Patent Laying-Open No. 59-081121) describes a multilayer pellet of a core-sheath structure prepared by, as a method of producing a pellet from an olefin-vinyl alcohol copolymer having such small melt tension that no strand is moldable for producing a pellet, molding a multilayer strand of a core-sheath structure formed by covering the outer periphery of a core of this resin having small melt tension with resin having large melt tension and thereafter cutting the same. As a production unit, that comprising a structure supplying a sheath material to the periphery of a core material on a die head for molding the pellet is shown.
In the method of producing a multilayer pellet disclosed in each of Patent Document 1 and Patent Document 2, the molding speed for the multilayer strand must be set relatively low, in order to reliably cover the outer periphery of the core material with the sheath material when molding the multilayer strand. Therefore, productivity of the multilayer strand and the multilayer pellet is so deteriorated that the production cost for the multilayer pellet is disadvantageously increased. In order to solve this problem, Patent Document 3 (Japanese Patent Laying-Open No. 2001-198918) discloses a method of producing a pellet by simultaneously producing a plurality of multilayer strands with a die assembly comprising a plurality of die heads, i.e., extrusion ports.
In the die assembly disclosed in Patent Document 3, a plurality of core material extruding passageways 1120 are provided on a circumference, as shown in FIG. 10A. Core material extruding passageways 1120 are provided with a plurality of radial feed pipes 1110 communicating with core material extruding passageways 1120 and extending in radial directions and a main feed pipe 1130 located on the centers of radial feed pipes 1110 for supplying a core material 910 thereto, for supplying core material 910 through these pipes.
In the die assembly disclosed in Patent Document 3, further, a plurality of sheath material extruding passageways 1220 are provided on the circumference, as shown in FIG. 10B. Sheath material extruding passageways 1220 are provided with a plurality of radial feed pipes 1210 communicating with sheath material extruding passageways 1220 and extending in radial directions and a main feed pipe 1230 located on the centers of radial feed pipes 1210 for supplying a sheath material 920 thereto, for supplying sheath material 920 through these pipes.
A plurality of multilayer strands can be simultaneously produced by forming extrusion portions with core material extruding passageways 1120 and sheath material extruding passageways 1220 and simultaneously extruding core material 910 and sheath material 920 from the plurality of extrusion portions. Core material extruding passageways 1120 and sheath material extruding passageways 1220 are provided on the circumference so that the same are supplied with core material 910 and sheath material 920 from radially extending plural feed pipes 1110 and 1210 respectively, whereby core material 910 and sheath material 920 are extruded from all core material extruding passageways 1120 and all sheath material extruding passageways 1220 under the same conditions.
Thus, dispersion in thickness etc. of core material 910 and sheath material 920 can be eliminated in all of simultaneously extruded plural multilayer strands 900.    Patent Document 1: Japanese Patent Laying-Open No. 07-171828    Patent Document 2: Japanese Patent Laying-Open No. 59-081121    Patent Document 3: Japanese Patent Laying-Open No. 2001-198918